In a nuclear reactor, moderated by means of light water, the fuel exists in the form of fuel rods, each of which contains a stack of pellets of a nuclear fuel arranged in a cladding tube, a column of extruded fuel cylinders of an uninterrupted column of vibration-compacted powdered fuel. The cladding tube is normally made of a zirconium-base alloy. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To keep the fuel rods at a distance from each other and prevent them from bending or vibrating when the reactor is in operation, a plurality of spacers are distributed along the fuel bundle in the longitudinal direction. A fuel assembly comprises one or more fuel bundles, each one extending along the main part of the length of the fuel assembly.
Together with a plurality of similar fuel assemblies, a fuel assembly is arranged in a core. The core is immersed in water which serves both as coolant and as neutron moderator. During operation, the water flows from below and upwards through the fuel assembly, whereby, in a light-water reactor of boiling water type, part of the water is transformed into steam. The percentage of steam increased towards the top of the fuel assembly. Consequently, the coolant in the lower part of the fuel assembly consists of water whereas the coolant in the upper part of the fuel assembly consists both of steam and of water. This difference between the upper and lower parts gives rise ot special factors which must be taken into consideration when designing the fuel assembly.
It is therefore desirable to achieve a flexible fuel assembly for a boiling water reactor which, in a simple manner, may be given a shape in which the upper part of the fuel assembly differs from the lower part thereof. A fuel assembly for a boiling water reactor with these properties is shown in PCT/SE95/01478 (Int. Publ. No. WO 96/20483). This fuel assembly comprises a plurality of fuel units stacked on top of each other, each comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate. The fuel units are surrounded by a common fuel channel with a substantially square cross section. A fuel assembly of this type may in a simple manner be given a different design in its upper and lower parts.
As in a light-water reactor of boiling water type, the water flows during operation from below and up through the fuel assembly of pressurized-water type. The temperature of the water increases the higher it rises in the assembly but it does not boil. A consequence of this is that corrosion of the cladding tubes increase sin the upper part as compared with the lower part of the fuel assembly. This difference between the upper and lower parts gives rise to special factors which must be taken into consideration when designing the fuel assembly.
It is therefore desirable, in the same way as has been described for a boiling water reactor, to achieve a flexible fuel assembly for a pressurized-water reactor which, in a simple manner, may be given a design in which the upper part of the fuel assembly differs from the lower part thereof. In UK 1 403 491, a fuel assembly for a pressurized-water reactor is shown, with a possibility of designing the upper part such that is differs from the lower part. This fuel assembly comprises a plurality of fuel units stacked on top of each other, each of which comprises a plurality of fuel rods extending between a top tie plate and a bottom tie plate. The fuel units are fixed to a centrally arranged support tube such that the bottom tie plate of one of the fuel units rests on the top tie plate of an adjacently arranged fuel unit and such that all the fuel rods in the fuel units are parallel to each other. The support tube extends through the whole fuel assembly and the fuel assembly has a substantially circular cross section. This fuel assembly is intended to be used in a nuclear reactor moderated by heavy water where the fuel assemblies are arranged in vertical pressure channels.
Another fuel assembly for a pressurized-water reactor with short fuel units with a hexagonal cross section is shown in "Improvements in Water Reactor Fuel Technology, Proceedings of a Symposium Stockholm, 15-19 September 1986, International Atomic Energy Agency, Vienna, 1987". The fuel units are shown vertically arranged and stacked to top of each other.
One factor which must be taken into consideration when designing a fuel assembly of light-water type is that the fuel rods grow to varying degrees during operation. The fuel assembly must therefore comprise members so as to allow this growth.
Another factor which must be taken into consideration is that the enthalpy across the core becomes as uniform as possible. For achieving enthalpy equalization across the core, it is known to provide fuel assemblies with a burnup-dependent flow resistance. In SE 460 452, elongated elements, such as fuel rods, are provided at their lower ends with restriction bodies which, when the elongated elements during irradiation are gradually, during the burnup of the fuel assembly, adapted to more and more restrict the flow of the coolant through an opening arranged below the restriction body by moving the restriction body closer and closer to this opening. In SE 9003330-9, the spacers are provided with members which are automatically, or manually, gradually activated during the burnup of the fuel assembly such that the coolant flow is deflected to one or more adjoining fuel assemblies.
The object of the present invention is to provide a fuel assembly for a light-water reactor comprising a plurality of short fuel units and members for differential growth of the fuel rods in the fuel units. Another object of the invention is to suggest a fuel assembly with a burnup-dependent flow resistance.